There has been a long term need for a better method of testing sandwich composites of the type used in applications such as in marine hulls, where the material must withstand loads which apply bends in the panel in two dimensions along two axes simultaneously over a substantial area of surface. While sandwich composite materials offer the potential to build structures which are lighter and stronger than ever before, there is a need for a practical standard test regimen that can realistically evaluate new hull composites and compare them with more traditional materials such as wood strip planking and marine plywood. There is a need for reliable data on the strength and durability of various materials which are contemplated for use by modern marine designers, and which must be considered by insurers and regulatory agencies.
In marine applications, particularly, unbacked expanses of sandwich composite materials are subjected to long term pressure loads or hydrostatic loading from the bodies of water in which they float. As propulsion takes place and waves are encountered, hydrodynamic loads are superimposed on the hull, and these loads induced by surface travel are repetitive and difficult to analyze. Because a typical hull is braced by intersecting bulkheads and stringers which form a lattice supporting the hull sheath, there are rectangular expanses of hull sheath spanning the interstices which are not directly braced, and water pressure forces these unsupported areas to bulge inwardly to some degree. Sandwich composites suitable for marine applications must be sufficiently stiff to resist the forces tending to bend the non-backed expanses in two dimensions along two axes simultaneously. Because bi-directional bending is involved, simple bending beam tests are not of great value in predicting the ultimate strength of the material or gauging sensitivity to visco-elastic effects, including time dependent phenomena such as creep rupture and fatigue. Particularly because such sandwich composites are nonhomogeneous and anisotropic, simple bending-beam tests do not recreate the complex interactions between resin, fiber, and core that take place when a flat composite panel is forced to compoundly bend.
In the past, too many hulls have been designed on the basis of static beam tests for flexural stiffness and, further, without recognition of the repeated decline in rigidity caused by repeated loads in the nature of fatigue cycles. This recognition is important in modern hull engineering, particularly with nonhomogeneous anisotropic materials.
For a number of years, various tests have been conducted on an MTS testing machine wherein a load measuring device or load cell is provided on an overhead crosshead, and a hydraulically operated platen is movable upwardly in this testing. Applicants' assignee has utilized a pyramidal plywood fixture carried by the load cell which had an open lower end of square configuration against which the test panel was pressed by a platen applied load transmitted to the lower surface of the test panel. A measuring device to effect the degree of bend applied to the specimen panel by the load was carried by the fixture, but difficulty was encountered in applying the contact load to obtain a better load distribution over the contact area. Feeling that it would be more appropriate to use hydrostatic pressure in the testing of hull materials, a water filled bladder was developed which could be supported on the platen and moved to bend the panel in the test. Such a bladder allowed significant panel deflection to take place while maintaining excellent load distribution. While the fixture assembly proved to be a useful tool for broad comparisons, it was not suitable for evaluations of a precise character because the edges of the panel projected beyond the pyramidal fixture and the corners of the panel tended to be displaced as a result of the bending load applied by the bladder.